Using ESR techniques we have recently demonstrated the presence of protein-centered free radicals in systems containing peroxidizing lipids and proteins. We have also shown that the free radical reactions lead to cross-linking of proteins and destruction of specific amino acid residues. We propose to extend these studies to determine the following aspects of reactions initiated by peroxidizing lipids in biological systems: 1. The nature of reactions leading to production of protein-centered free radicals. 2. The important lipid species (whether lipid hydroperoxides or secondary oxidation products) responsible for the radical transfer. 3. Effects of environmental factors such as water activity and the presence of anti- or pro-oxidants on the transfer of free radicals from peroxidizing lipids to proteins. 4. The relevance of general mechanisms elaborated on model systems to behavior of lipid-protein interactions in more complex tissues. The overall objective of the proposed research is the understanding of the mechanisms and conditions of occurrence of free radical reactions between peroxidizing lipids and other biological materials, especially proteins. A further aim is to develop procedures leading to the prevention or minimization of undesirable reactions in biological systems and in stored foods.